Cable television networks such as those provided by Comcast Cable Communications, Inc., of Philadelphia, Pa., Cox Communications of Atlanta, Ga., Tele-Communications, Inc., of Englewood Colo., Time-Warner Cable, of Marietta Ga., Continental Cablevision, Inc., of Boston Mass., and others provide cable television services to a large number of subscribers over a large geographical area. The cable television networks typically are interconnected by cables such as coaxial cables or a Hybrid Fiber/Coaxial (“HFC”) cable system which have data rates of about 10 Mega-bits-per-second (“Mbps”) to about 30+Mbps.
The Internet, a world-wide-network of interconnected computers, provides multi-media content including audio, video, graphics and text that typically requires a large bandwidth for downloading and viewing. Most Internet Service Providers (“ISPs”) allow customers to connect to the Internet via a serial telephone line from a Public Switched Telephone Network (“PSTN”) at data rates including 14,400 bps, 28,800 bps, 33,600 bps, 56,000 bps and others that are much slower than the about 10 Mbps to about 30+Mbps available on a coaxial cable or HFC cable system on a cable television network.
With the explosive growth of the Internet, many customers have desired to use the larger bandwidth of a cable television network to connect to the Internet and other computer networks. Cable modems, such as those provided by 3Com Corporation, of Santa Clara, Calif., Motorola Corporation, of Arlington Heights, Ill., Hewlett-Packard Co., of Palo Alto, Calif., Scientific-Atlanta, of Norcross, Ga., General Instruments, of Horsham, Pa., and others offer customers higher-speed connectivity to the Internet, an Intranet, Local Area Networks (“LANs”) and other computer networks via cable television networks. These cable modems currently support a data connection to the Internet and other computer networks via a cable television network with a data rate of up to about 30+Mbps, which is a much larger data rate than can be supported by a modem used over a serial telephone line.
However, many cable television networks provide only unidirectional cable systems, supporting only a “downstream” cable data path. A downstream data path is the flow of data from a cable system “headend” to a customer. A cable system headend is a central location in the cable television network that is responsible for sending cable signals in the downstream direction. A return data path via a telephone network (i.e., a “telephony return”), such as, a Public Switched Telephone Network provided by AT&T, GTE, Sprint, MCI and others, is typically used for an “upstream” data path. An upstream data path is the flow of data from the customer back to the cable system headend. A cable television system with an upstream connection to a telephony network is called a “data-over-cable system with telephony return.”
An exemplary data-over-cable system with telephony return includes customer premise equipment (e.g., a customer computer), a cable modem, a cable modem termination system, a cable television network, a Public Switched Telephone Network, a telephony remote access concentrator and a data network (e.g., the Internet). The cable modem termination system and the telephony remote access concentrator together are called a “telephony return termination system.”
The cable modem termination system receives data packets from the data network and transmits them downstream via the cable television network to a cable modem attached to the customer premise equipment. The customer premise equipment sends response data packets to the cable modem, which sends response data packets upstream via Public Switched Telephone Network to the telephony remote access concentrator, which sends the response data packets back to the appropriate host on the data network.
In a two-way cable system without telephony return, the customer premise equipment sends response data packets to the cable modem, which sends the data packets upstream via the cable television network to the cable modem termination system. The cable modem termination system sends the data packets to appropriate hosts on the data network. The cable modem termination system sends the response data packets back to the appropriate cable modem. Currently, as a cable modem is initialized in a data-over-cable system, it registers with a cable modem termination system to allow the cable modem to receive data over a cable television connection and from a data network (e.g., the Internet or an Intranet). The cable modem forwards configuration information it receives in a configuration file during initialization to the cable modem termination system as part of a registration request message. A cable modem also helps initialize and register any attached customer premise equipment with the cable modem termination system.
A cable modem termination system in a data-over-cable system typically manages connections to tens of thousands of cable modems. Most of the cable modems are attached to host customer premise equipment such as a customer computer. To send and receive data to and from a computer network like the Internet or an Intranet, a cable modem and customer premise equipment and other network devices have a network address dynamically assigned on the data-over-cable system.
Many data-over-cable systems use a Dynamic Host Configuration Protocol (“DHCP”) as a standard messaging protocol to dynamically allocate network addresses such as Internet Protocol (“IP”) addresses. As is known in the art, the Dynamic Host Configuration Protocol is a protocol for passing configuration information to the network devices on a network. The Internet Protocol is an addressing protocol designed to route traffic within a network or between networks.
The cable modem makes an Internet Protocol connection to the cable modem termination system so that Internet Protocol data received on the cable modem termination system from the data network can be forwarded downstream to the customer premise equipment via the cable network and the cable modem. Once an Internet Protocol address is obtained on the cable modem termination system, the cable modem obtains the name of a configuration file used to complete initialization. The cable modem downloads a configuration file from a central location in the data-over-cable system using a Trivial File Transfer Protocol (TFTP) server. As is known in the art, Trivial File Transfer Protocol is a very simple protocol used to transfer files, where any error during file transfer typically causes a termination of the file transfer.
There are a host of initialization steps that are typically performed to allow the network client device such as a cable modem to receive data over a cable television connection from a data network. A set of parameters must be initialized before the cable modem can be declared operational. Some of these parameters include synchronization, authorization, local address assignment, ranging and power calibration, assignment of default upstream and downstream channels and assignment of encryption information.
Ranging is a process by which the headend determines the round-trip delay of data destined to a specific customer premise equipment or network termination point. It is a process of acquiring the correct timing offset such that the cable modem's transmissions are aligned to the correct mini-slot boundary. Accurate ranging of network client devices permits a Time Division Modulation Application (TDMA) like slotted channel mechanism on the upstream. Further, less guard time is required between network client devices such as a cable modem with precise ranging. All network client device transmitters along the entire length of the cable television system are aligned in terms of timing such that, if every network client device on the channel began transmitting on the upstream channel, the first symbol of each would arrive at the headend receiver at exactly the same instant the first symbol of the downstream frame was leaving the headend transmitter.
During the ranging process, each network client device is transmitter downloaded with a transmit timing offset value. IEEE P802.14 specifies a maximum cable television length of 50 miles (80 km). The ranging process has the effect of positioning each network termination point in a virtual timing space such that all network client devices appear to be within zero propagation delay of the headend. For example, using the difference between its current time and a cable modem's local time at the time a frame is transmitted, the headend can determine a time correction value to be assigned uniquely to each cable modem. The INVITATION and INVITATION—RESPONSE messages between a cable modem and a headend exchange all the data necessary to perform ranging, but additional ranging exchanges may be performed on any cable modem via the RANGE—REQUEST message. The range and power calibration message from the headend to a cable modem is transmitted by the headend to a single cable modem on the cable modem's downstream channel after it has transmitted a local address assign message to the same cable modem. It contains the range offset value and the power control value and is addressed to the cable modem's local address. This message may be transmitted to the same cable modem in response to any received upstream slot to cause it to adjust its power and range offset.
The headend also determines a power level adjustment value to guarantee optimal performance. However, the Data-Over-Cable Service Interface Specification (DOCSIS) provides no guidance as to how the power level should be adjusted for successive initial ranging attempts. The specification only specifies a dynamic range from 8 dBmV to 58 dBmV with power level control in 1 dB increments. One solution may be to use linearly increasing increments in power to cover a transmitter dynamic range. The problem with this approach is the considerable length of time required by a cable modem to register with the cable modem termination system. Thus, there is still a need to implement a method to calibrate or adjust the power level of a cable modem transmitter that reduces the initial ranging time to cover the entire cable modem transmitter dynamic range.